1. Field of the Invention
The present invention relates to ultrasonic imaging and more particularly to the formation of an area array of ultrasonic transducers which senses points of a sonic wavefront with adequate resolution to form a useful image. The arrays herein disclosed have application to the imaging of small objects such as are of interest in medical application as well as to the imaging of larger distant objects in a waterborne environment.
2. Description of the Prior Art
Ultrasonic imaging is of growing interest. Its two principal spheres of application are to medical electronics and to underwater viewing. In its medical application, ultrasonic imaging acts as a substitute and supplement to x-ray examination. It has the advantage that at the levels of ultrasonic energy normally required, the possibility of injury to the patient is remote. At the same time, ultrasonic examination supplements x-ray examination since it permits one to obtain information under conditions where x-radiation cannot be used. Normal ultrasonic examination relies on the relatively good contrasts which exist between body fluids, soft tissues and bone. In underwater applications, the ultrasonic viewing has the advantage of having greater water penetration than other forms of wave energy, and presents the possibility, not yet fully realized, of imaging objects with relatively high resolution at relatively great distances in the water.
While practical usage is going on in both of the foregoing applications, measured by the high performance standards set by x-radiation equipment in medical applications and high resolution radar systems for distant viewing, ultrasonic imaging systems are in a primitive stage. In medical applications, the principal mode of ultrasonically examining a sample is to use a single scanned transducer for both transmission and sensing. The information is then supplied to a storage tube in such a way as to create a slowly formed display of the area under examination. The system is usable but much less than optimum in relation to resolution and the speed by which images are recreated. Like limitations exist in underwater viewing operations.
A critical element in such ultrasonic imaging systems is the means by which the sonic fields are sensed in order to form an image. Scanned single transducers are in use and being refined. Arrays of large numbers of transducers have been proposed to take their place. That arrays, which would sense a large number of points in the sonic field simultaneously are preferable, is unquestioned. Arrays of transducers are used in sonar systems and analagous uses are being made of large numbers of elements in high resolution radar systems. In ultrasonic applications, the arrays presently known suffer from poor resolution and from constructional complexity. A primary problem has been the inability to assemble the individual transducers making an array with sufficient density to obtain all the information in the sonic field and at the same time decouple adjacent array elements. A second problem has been the prohibitive cost of making the large numbers of elements, which such arrays would normally require.
One known array for ultrasonic imaging has been described in the literature using resonant elements supported upon a rigid plate. These elements were designed to operate with their free ends as antinodes and their supports as nodes. In practice, there does not appear to be a truly rigid plate. The stresses occurring from unbalanced vibration of one resonator deform the plate and the plate couples the vibrations from one resonator to another. This coupling destroys the resolution that would otherwise be predictable from the design parameters.